1. Field of the Invention
The present invention relates to an apparatus which optically checks paper sheets such as bank notes. In particular, this invention relates to the apparatus which detects permeated light and reflected light by using multiple optical sensors, and carries out a check based on the detected output.
2. Description of the Related Art
In optically checking paper sheets such as, for example, bank notes, a check is carried out to determine whether a pattern printed on the paper surface is correct. In many cases, the paper sheets are checked by using multiple elements. The outputs of these elements must be made constant.
One conventional method of making the outputs constant is to provide a volume to each of the detecting circuits in the elements of the optical sensor, and to match the detecting levels of all the elements by adjusting their volumes.
However, adjustment is extremely complex when there is a great number of elements.
In another conventional method, disclosed for example in Japanese Patent Application No. 61-4819, the sensitivity level of the optical sensor is set at the initial stage in order to determine whether or not a medium exists.
However, this optical sensor only determines whether or not a medium exists. When applied to paper sheets, the optical sensor creates a two-value signal for detecting but cannot create a measuring signal.
This invention has been realized in consideration of the above points, and aims to provide an optical paper sheet detecting apparatus capable of measuring with stability and optimum precision light which has been permeated through or reflected from a paper sheet in order to detect optical characteristics thereof.
In order to achieve the above objects, the optical paper sheet checking apparatus of this invention comprises a plurality of optical sensors which output signals obtained by optically detecting a pattern of a paper sheet, these optical sensors comprising light emitting elements and light-receiving elements; a driving unit which supplies a current to the light emitting elements in the plurality of optical sensors in accordance with a control signal; and a controlling unit. While the paper sheet is not in a predetermined position, the controlling unit supplies a maximum current having a predetermined value to the light emitting elements. Thereafter, the controlling unit supplies the control signal to the driving unit so as to decrease the supply of current, and determines and stores optimum values for each of the plurality of optical sensors. The optimum value is deemed to be the value of current supplied when the output voltage of the light-receiving elements has changed by a predetermined value in proportion to its value at the maximum current. When the paper sheet is at the predetermined position, the controlling unit supplies the control signal to the driving unit based on each of the optimum values which have been stored, selectively extracts the detected signals from the optical sensors and outputs detected data representing the pattern of the paper sheet.